A cathode ray picture tube is an apparatus for displaying images which forms an electron beam by focusing and accelerating electrons generated from a cathode, and makes the beam selectively scan a phosphor screen by deflection means.
An electron gun is used to form the electron beam. An electrostatic type gun is typically used which focuses the beam by electrostatic lenses formed by the potential difference between a plurality of grids to each of which different voltages are applied.
At each of the grids of the electron gun, there are formed beam passing holes through which electron beams to be controlled are passed. In a Black and White (B/W) gun, circular holes and accordingly circular lenses are generally adopted.
As for a gun for a color picture tube having three beam passing holes, a large amount of spherical aberration and astigmatism occurs, as boundary conditions for each of the R, G, B beams are different from each other. A main lens of a high brightness color picture tube, for example, is formed in a large diameter lens in which all three beams pass through a large common aperture to reduce the spherical aberration. Because of its shape, the main lens of this type has a built-in astigmatism which deforms the landing spot and deteriorates the resolution of the picture tube.
The astigmatism occurs when the horizontal component and the vertical component of the electron beam, namely the horizontal beam and the vertical beam, are respectively focused in different intensities. A positive astigmatism means a stronger focusing by the vertical beam than the horizontal beam, and a negative astigmatism means the contrary.
The astigmatism can be compensated by fabricating at least one quadrupole lens having an opposite astigmatism to the electron gun main lens. Comparing with the circular lens which focuses all beams to a point, the quadrupole lens is used as a generic name of a non rotational-symmetric lens which focuses the electron beam differently in two perpendicular planes.
Meanwhile, means for deflecting, such as a deflection yoke, is used for scanning the electron beam. A self convergence type yoke forms a yoke field having a positive astigmatism which causes the deflection defocusing and distort distorts the shape of beam spots. In other words, the vertical component of the electron beam, namely the vertical beam, is overfocused by the positive astigmatism of the deflection yoke. This overfocusing can be compensated by adopting a dynamic focusing gun. In the gun, intensities of the main lens and the quadrupole lens are in synchronism with the deflection, to focus and reform the beam spot to be circular again.
Referring to FIG. 1, a typical quadrupole lens is formed by providing vertically or horizontally elongated electron beam passing holes Hv, Hh to each of two adjacent grids E1', E2'. Each of holes Hv, Hh of two grids E1', E2' are vertically or horizontally formed in parallel. When a vertically elongated hole is formed on a high potential lens or when a horizontally elongated hole is formed on a low potential lens, the quadrupole lens formed therebetween has a positive astigmatism. Assuming the E1' designates a high potential grid, and E2' a low potential grid, for example, a quadrupole lens of positive astigmatism is formed between two grids E1', E2'. Meanwhile, flanges f are formed to control the shape of the electric field, at outer edge portions of holes Hv, Hh.
These grids E1', E2' are assembled to an electron gun by being secured to glass rods, which are commonly called bead glasses, through heat melting. During the assembly, each of grids E1', E2' are temporarily secured by the insertion of mandrels of a jig to each of beam passing holes Hv, Hh, and then assembled with bead glasses.
Referring to FIG. 2, three mandrels P are fabricated on a jig(not shown), and each of component grids E1', E2' are sequentially inserted thereon. As most of the grids of the gun have circular beam passing holes, mandrels P are also formed in circular section. And the diameter of each mandrel is stepwisely reduced toward its end, as diameters of beam passing holes become smaller, closer to the cathode.
Meanwhile, grids E1', E2' for forming a quadrupole lens, have vertically or horizontally elongated holes Hv, Hh, and cannot be secured to mandrels P of circular sections. As depicted in FIG. 2, the upper grid E1' having vertically elongated holes Hv moves up and down, and the lower grid E2' having horizontally elongated holes Hh moves right and left, thereby injuring the alignment of the assembled gun. The misalignment of the gun not only deteriorates the color purity and the resolution, but also distorts the electrostatic lens. To prevent this problem, an additional precise assembly hole and guide pin, or an external guiding means are needed.